U.S. patent number 4,734,117 [Application Number 07/042,567] was granted by the patent office on 1988-03-29 for optical waveguide manufacture.
This patent grant is currently assigned to Northern Telecom Limited. Invention is credited to Richard S. Lowe, Peter J. Pilon.
United States Patent |
4,734,117 |
Pilon , et al. |
* March 29, 1988 |
Optical waveguide manufacture
Abstract
Optical waveguide having a fused silica core and a fluorine
doped fused silica cladding is made by depositing particulate core
silica onto a support tube and then drying and densifying the
silica. Further particulate cladding silica is deposited and is
heated in a fluorine containing gas to effect drying, fluorine
diffusion into and sintering the cladding silica. The support tube
is etched away and the resulting tubular preform is heated to
collapse it into a rod from which waveguide is drawn, the waveguide
having a fluorine doped silica cladding.
Inventors: |
Pilon; Peter J. (Nepean,
CA), Lowe; Richard S. (Kanata, CA) |
Assignee: |
Northern Telecom Limited
(Montreal, CA)
|
[*] Notice: |
The portion of the term of this patent
subsequent to February 17, 2004 has been disclaimed. |
Family
ID: |
4130060 |
Appl.
No.: |
07/042,567 |
Filed: |
March 30, 1987 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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745778 |
Jun 17, 1985 |
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Foreign Application Priority Data
Current U.S.
Class: |
65/397; 65/399;
65/426; 65/428; 65/435; 65/DIG.16 |
Current CPC
Class: |
C03B
37/014 (20130101); C03B 37/01446 (20130101); C03B
37/01473 (20130101); C03B 37/01486 (20130101); C03B
37/027 (20130101); G02B 6/102 (20130101); C03B
37/01493 (20130101); Y10S 65/16 (20130101); C03B
2201/12 (20130101) |
Current International
Class: |
C03B
37/027 (20060101); C03B 37/02 (20060101); C03B
37/014 (20060101); G02B 6/10 (20060101); C03P
037/018 () |
Field of
Search: |
;65/3.12,DIG.16,3.11,3.15,18.2,900 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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3031160 |
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Apr 1982 |
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DE |
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3230199 |
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Feb 1984 |
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DE |
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2504514 |
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Oct 1982 |
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FR |
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55-67533 |
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May 1980 |
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JP |
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56-50136 |
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May 1981 |
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JP |
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Primary Examiner: Schor; Kenneth M.
Attorney, Agent or Firm: Schwartz, Jeffery, Schwaab, Mack,
Bleumenthal & Evans
Parent Case Text
This application is a continuation of application Ser. No. 745,778
filed June 17, 1985, now abandoned.
Claims
What is claimed is:
1. A method of manufacturing optical waveguides comprising:
depositing a layer of particulate silica on an outer surface of a
fused silica cylindrical support tube having a central bore;
drying the deposited silica;
consolidating the dried, deposited silica;
depositing another layer of particulate silica on the dried
consolidated silica to form a preform having a porous outer
layer;
placing said preform having a porous outer layer within a second
fused silica tube, and drying and fluorine doping said preform
having a porous outer layer by passing a fluorine-containing gas
through said second fused silica tube and around said preform
having a porous outer layer to form a preform having a doped porous
outer layer;
consolidating said preform having a doped porous outer layer to
form a preform having a doped outer layer and heating said second
fused silica tube to collapse it onto said preform having a doped
outer layer;
etching away the fused silica cylindrical support tube by passing a
mixture of hexafluoride and helium through the central bore,
resulting in a tubular preform composite;
collapsing said composite tubular preform into a rod preform;
and
heating said rod preform to a drawing temperature and drawing it
into an optical waveguide, said waveguide having a core and a
cladding derived from deposited silica, with the cladding being
doped with fluorine.
2. A method as claimed in claim 1 in which the support tube is made
of fused silica doped with a material which renders the silica more
easily etchable than pure silica.
3. A method as claimed in claim 1 in which etching is continued
after removal of the support tube so as to etch away a portion of
the deposited silica on the outer surface of the fused silica
cylindrical support tube.
4. A method as claimed in claim 1 in which the support tube is
mounted between spaced chucks during deposition of particulate
silica on the outer surface of the fused silica cylindrical support
tube.
5. A method as claimed in claim 1 in which during the step of
depositing a layer of particulate silica on an outer surface of a
fused silica cylindrical support tube, a refractory liner rod is
positioned within the support tube.
6. A method as claimed in claim 1 in which during the first step of
consolidating, a refractory liner is positioned within the support
tube.
Description
The invention relates to a method for manufacturing optical
waveguide. It has particular application to the manufacture of
optical waveguide having a fluorine doped silica cladding and a
pure or doped silica core.
Optical waveguide having a pure silica core and a fluorine doped
silica cladding is described in U.S. Pat. No. 4,082,420 (Shiraishi
et al). The optical waveguide is made by a flame hydrolysis method
in which silicon tetrachloride and silicon tetrafluoride are fed to
an oxygen-hydrogen burner to form a flourine doped silica soot
which is deposited onto the surface of a vitreous silica rod. The
rod and deposited soot are then heated to consolidate the soot into
a composite glass preform and fiber is drawn from the preform.
Using the flame hydrolysis method, it has proven difficult to
entrain sufficient fluorine into the deposited silica. The fluorine
lowers the refractive index of the silica but the dopant content in
the cladding must be sufficient to lower the refractive index of
silica from about 1.4585, being that of pure silica, to about
1.4550 or less in order that a fiber having a pure silica core and
a doped silica cladding will function as a waveguide.
Our copending Canadian patent application Ser. No. 476,843, in the
name of Koichi Abe and entitled OPTICAL WAVEGUIDE MANUFACTURE,
describes an alternative method of fabricating a fluorine doped
silica clad fiber. In the method a fluorine doped silica cladding
is made by heating a cylinder of silica in a fluorine-containing
atmosphere, the cylinder initially having an outer annular
particulate or porous region deposited over a core region of fused
silica. The fluorine diffuses into this porous annular region to
both dry and lower the refractive index of the porous silica which
is subsequently fused. The cylinder is made by depositing
particulate core silica onto a mandrel, removing the mandrel,
drying the silica in chlorine, and heating the silica to densify
it. Further particulate silica is deposited and is then heated in a
fluorine-containing gas to dry, fluorine diffuse, and sinter the
porous outer part of the silica. The resulting tubular silica
preform is heated to collapse the tubular preform into a rod from
which optical waveguide is drawn, the waveguide having a fluorine
doped silica cladding. The particular silica can be subjected to a
chlorine drying step before fluorine drying, diffusion and
sintering.
A preferred mandrel was composed of a graphite rod, the rod being
removed after depositing the core silica. The rod was removed by
twisting the deposited silica tube to break the seal between the
tube and the mandrel, the mandrel then being withdrawn along the
axis of the tube.
One disadvantage of using a graphite mandrel is that particles of
graphite can remain within the deposited silica tube and impurities
out-diffusing from the graphite during subsequent heating steps can
impregnate the silica tube. The same disadvantage results from
using a carbon coated rod which has been proposed as an
alternative. A further problem in the method previously disclosed
is that the initially deposited thin-walled core silica tube can be
distorted in shape since the supporting graphite mandrel is absent
during the sintering step when the temperature reaches about
1600.degree..
To overcome these disadvantages there is proposed according to the
present invention a method of manufacturing optical waveguide
comprising depositing particulate core silica onto a cylindrical
mandrel, drying the particulate core silica and then fusing the
silica to form a cylindrical silica substrate, forming a further
layer of particulate cladding silica on the cylindrical silica
substrate, diffusing fluorine into the particulate cladding silica
layer, heating the silica to cause consolidation and collapse of
the core and cladding silica into a fused rod preform, heating the
rod preform to a drawing temperature and, drawing optical waveguide
from the rod preform, such waveguide having a cladding part derived
from the deposited particulate cladding silica and a core part
derived from the substrate silica, the improvement comprising said
mandrel being a fused silica support tube which is etched away
prior to collapse of the silica into a fused rod preform by passing
an etchant within the support tube.
In a preferred embodiment, the silica support tube remains until
immediately before consolidation and collapse into the fused silica
preform. Particularly in manufacturing single mode waveguide a
outer silica tube is collapsed down onto the preform after
sintering the cladding silica. The preform obtained is then
relatively massive and the processes of etching away the initial
support tube and collapsing the tubular preform to a rod are
relatively slow because of the problem of heating the center of the
preform. The support silica tube can be etched using a silica
etchant such as a mixture of sulphur hexafluoride and helium. As
the etchant is passed through the support tube the support tube
together with the deposited silica is subjected to passes of a
burner flame. Using the embodiment, there is no risk of introducing
moisture into the core and cladding while they are porous or into
the cladding surface when sintered since the outer silica tube
protects the core and cladding regions from the wet flame during
the final steps.
The support tube can alternatively be etched away at an earlier
stage such as immediately after consolidation of the
first-deposited or core silica. The core silica can then be
collapsed into a rod before deposition, drying and fluorine
diffusion of the outer or cladding silica. Also, at an early stage
of processing, the quality and thickness of the core silica can be
assessed to see whether it is worth depositing further cladding
silica and to determine how much cladding silica should be
deposited to obtain a desired core/cladding thickness ratio.
However in this alternative the cylinder of core silica must be
relatively large to be self supporting. As indicated previously all
high temperature processing steps other than silica deposition are
preferably implemented within a tubular silica chamber to which the
desired ambient temperature is supplied and heat is applied to the
outside of that chamber rather than directly to the deposited
silica. It is difficult to accommodate a large core cylinder in
such a chamber and still develope a high enough temperature for
sintering to occur. The cylinder of core silica must therefore be
heated directly to consolidation using an oxygen-hydrogen torch.
Unfortunately use of direct heating introduces the highly
attenuating OH group into an outer layer of the core silica
cylinder and this layer must be etched away before subsequent
processing using a HF:H.sub.2 O etchant.
One embodiment of the invention will now be described with
reference to the accompanying drawings in which:
FIG. 1 shows the end part of an optical waveguide made by a method
according to the invention, the Figure also illustrating a
refractive index profile across the fiber; and
FIGS. 2 to 8 are schematic views of stages in the manufacture of
optical waveguide by one method according to the invention.
Referring to FIG. 1, an optical fiber has a core 10 of high purity
fused silica, a cladding 12 of fluorine doped silica, and a silica
jacket 13. The optical fiber has an outer diameter of 125 microns
with a core diameter of about 9 microns for single mode fiber and
about 50 microns for multimode fiber. The fluorine is present in an
amount sufficient that the refractive index of the cladding region
is 1.4550 or less compared to 1.4585 for the core region.
To make a fiber having the structure and composition shown in FIG.
1, a cylindrical preform is made from particulate silica. The
preform is dried and fluorine is diffused into an outer porous
region of the preform. The preform is then consolidated into a
fused silica rod from which fiber is drawn, the fiber having a
relatively low refractive index cladding corresponding to the
fluorine doped region.
Referring particularly to FIGS. 2 to 8, FIG. 2 shows a tubular
fused silica support tube 14, 50 centimeters in length with an
internal diameter of 4 millimeters and an external diameter of 6
millimeters. The ends of the silica support tube are fixed into
spaced chucks 17 of a lathe. A silica soot producing burner 18 is
mounted to direct a flame at the support tube 14. Silicon
tetrachloride entrained within a stream of oxygen by bubbling the
oxygen through the silicon tetrachloride is fed to a central
tubular chamber within the burner 18. Argon, which separates the
silicon tetrachloride vapour from the burner gases within the
burner itself is fed to a second surrounding annular chamber,
hydrogen to a third annular chamber, and a mixture of argon and
oxygen is fed to an outer burner chamber. The flow rates are 2 to 3
liters per minute of oxygen to the first chamber, 2 liters per
minute of argon to the second chamber, 10 liters per minute of
hydrogen to the third chamber and 15 liters per minute of argon
with 3 liters per minute of oxygen to the outer chamber. The burner
is moved along the length of the support tube at 8 centimeters per
minute and the support tube 14 is rotated at 30 revolutions per
minute.
Particulate core silica is deposited onto the support tube 14 to a
diameter of 1.2 centimeters and at a rate of growth which depends
on the diameter of the support tube as supplemented by previously
deposited particulate silica. The deposited silica has a very high
moisture content. This is untenable if the silica is to function as
the core of an optical waveguide since the moisture results in a
large absorption peak near 1400 nanometers. This reduces the
transmission at 1300 and 1550 nanometers which are the output
wavelengths of long wavelength light emitting devices of interest
in fiber optic communications systems.
To remove this OH moisture absorption peak, the particulate silica
is dried in a chlorine-containing atmosphere at high temperature.
As shown in FIG. 3, the support tube 14 together with deposited
silica 21 is mounted within a 16 millimeter internal diameter
silica tube 28 using apertured Teflon (Trademark) spacers 19 which
permit the tube 28 to be rotated with the support tube 14 held
centrally. A mixture of chlorine (200 cubic centimeters per minute)
and helium (200 cubic centimeters per minute) is then piped through
the tube 28 and a burner flame 20 is directed at the outside of the
tube 28 to establish a hot zone temperature of 1300.degree. C. The
torch is passed several times along the tube 28 in the direction of
flow of the gas mixture. Torch traversal takes place at 8
centimeters per minute for a time of 60 minutes. During this period
the porous silica 21 shrinks to about 0.9 centimeters in diameter
corresponding to densification from an initial value of about 0.35
grams cm.sup.-3 to a final density of about 0.8 grams cm.sup.-3.
Hydrogen contained within the porous silica as the hydroxyl species
reacts with the chlorine to produce volatile hydrogen chloride and
is removed. Excess chlorine and hydrogen chloride are exhausted
from the tube leaving only chlorine within the particulate silica.
Removal of hydroxyl species renders subsequently formed fused
silica very highly transmissive.
In a subsequent sintering or consolidation step, the burner
traversal rate is reduced to 0.2 centimeters per minute and the gas
applied to the burner is altered to obtain a hot zone temperature
of about 1600.degree. C. After a one hour burner traversal period,
the soot is consolidated to a chlorine free fused silica tube 27
about 30 centimeters in length having an external diameter of 0.65
centimeters. A back pressure generating device can be used at the
exhaust end of the tube 28 to prevent the tube from shrinking in
diameter at high temperature.
Referring to FIG. 4, the silica tube 14 is mounted between quartz
chucks 17 and further particulate silica 22 is deposited onto the
densified silica 27 using the burner 18. The particulate silica 22
is deposited to a diameter of 2.4 centimeters with a density of
0.35 grams cm.sup.-3.
When silica deposition is complete, the support tube 14, which
supports the core and cladding silica, is placed within a fused
silica tube 23 having an inside diameter of 2.8 centimeters and a 1
millimeter wall thickness (FIG. 5). The tube is mounted between
apertured Teflon discs 24 which permit tube rotation. The porous
silica 22 is then simultaneously dried and doped with fluorine by
passing along the tube 23 a mixture of helium (180 cubic
centimeters per minute) and sulphur hexafluoride (45 cubic
centimeters per minute). A single burner pass is made in the
direction of gas flow at a traversal rate of 0.4 centimeters per
minute and a hot zone temperature in the range 1450.degree. to
1550.degree. C. Because of the presence of fluorine in the porous
silica, the sintering temperature is lower than that of pure
silica. Consequently the heat pass not only dries and dopes the
silica but causes sintering as shown at region 25. A fused silica
tube is produced having an external diameter of 1.1 centimeter. An
outer annular region of the tube is doped with fluorine to a level
at which the doped silica refractive index is 1.4520 compared to
1.4585 for the pure silica in the central region. The refractive
index difference of 0.0065 is suitable for making multimode optical
waveguide.
Although it is convenient to perform the drying and fluorine doping
steps simultaneously, the steps can in fact be performed
successively in which case an alternative drying agent such as
chlorine can be used. By using the chlorine and fluorine drying
techniques at various stages in the fabrication process, a moisture
level of less than 0.1 parts per million in the fused silica is
achieved.
The diameter of the tubular preform is then increased from about
1.1 centimeter to about 1.5 centimeters by adding the silica jacket
13 (FIG. 1). To do this the tube 23 in which the preform is mounted
is simply heated to collapse it down onto the outside of the
fluorine doped silica 25. As shown in FIG. 6, the tube 23 collapses
along a central region but is maintained open at its ends. The
composite preform obtained has a small bore 29 through its center.
As shown in FIG. 7, a mixture of sulphur hexafluoride and helium in
the volume ratio 10%:90% is then passed down bore 29. At the same
time, the preform is subjected to a number of passes of the burner
20 at about 1 centimeter per minute to establish a temperature at
the center of the preform of about 1200.degree. C. At this
temperature the silica of the initial support tube 14 is etched
away. To accurately gauge the amount of material removed and to
prevent the removal of deposited core silica 27, the preform is
periodically weighed or the cross-section is optically monitored.
Once the initial support tube 14 has been fully etched away, the
composite tubular preform is collapsed by heating to a temperature
in the range 1850.degree. to 1900.degree. C. and traversing the
burner at 1 centimeter per minute towards the inlet end while
maintaining a clean helium atmosphere in the bore 29.
The composition of the support tube 14 can be silica doped, for
example, with the OH group to improve etchability. However care
must be taken to ensure that impurity is not absorbed into the core
and cladding silica 27 and 35 while they are porous. Moreover any
impurity used should not cause a marked change in the coefficient
of thermal expansion of the silica since otherwise there will be
untenable stress introduced at the support tube/core silica
interface during temperature fluctuations associated with
processing.
As indicated previously the support tube 14 can be etched away at
an earlier stage in the process. The core silica 27 can then be
collapsed into a rod to form a substrate for cladding silica
deposition. The slow, high temperature step of collapsing the
central bore 29 of the full size tubular preform is thus
obviated.
As shown in FIG. 2, in phantom, a 4 millimeter outside diameter rod
made of a refractory material such as graphite or alumina (Al.sub.2
O.sub.3) can be temporarily inserted into the tube 14 to reduce
warpage. The presence of such a refractory liner permits high
temperature of the core silica to be achieved without destroying
the cylindrical nature of the preform.
Referring to FIG. 8, the composite rod preform finally obtained is
subsequently placed in a vertical orientation drawing tower having
a furnace zone at which the preform temperature is raised to about
2000.degree. C. which is higher than the silica softening point.
Fiber is pulled from the lower end of the preform by a drum onto
which the fiber is wound after being cooled and coated with a
protective acrylate or silicone layer. The fiber has a high purity
silica core and a relatively lower refractive index fluorine doped
silica cladding.
The waveguide produced using the dimensions indicated is multimode
fiber having a core diameter of about 50 microns, a combined
cladding and jacket diameter of about 80 microns, and an overall
diameter of 125 microns. As previously indicated the refractive
index difference is approximately 0.007. Monomode fiber, in
contrast has a smaller core of the order of 9 microns in diameter
and a refractive index difference of approximately 0.0035. To
modify the method for monomode fiber, the core:cladding diameter
ratio is reduced. One way of achieving this is merely by depositing
more particulate cladding silica at the stage described with
reference to FIG. 4. Another method is to etch away some of the
core silica bounding the bore 29, following removal of the support
tube 14. A refractive index difference of the order of 0.0035 is
achieved by doping the porous cladding silica to a lower level by
increasing the ratio of helium in the helium:sulphur hexafluoride
mixture used during the fluorine drying/doping stage of FIG. 5. The
amount of fluorine necessary to lower the refractive index of
silica to 1.452 is about the limiting level at which fluorine can
be incorporated into silica using this method. To obtain a
refractive index difference larger than 0.007 for a silica based
fiber, the refractive index of the core can be increased above the
value of that of the pure silica. Most dopants increase the
refractive index of silica, so by incorporation of the dopant
material within the silica initially deposited onto the support
tube 14 the refractive index of the resulting waveguide core is
increased above that of pure silica. Germanium can be included
within the silica initially deposited by entraining germanium
tetrachloride with silicon tetrachloride injected into the silica
soot producing burner 18.
As described in our co-pending Canadian application Ser. No.
463,378, the core and cladding silica can be deposited in other
ways. For example the porous cladding silica can be deposited
immediately following the porous core silica with a density less
than that of the core silica. Following chlorine drying of all the
deposited silica, the cladding silica is subjected to fluorine
drying, diffusion and sintering. Because the core silica is
deposited in a relatively dense condition, the fluorine diffuses
only into the cladding silica. In other respects the method is the
same as that described previously.
In a further alternative process, the core and cladding particulate
silica is deposited with uniform porosity onto the fused silica
support tube. The preform is dried in chlorine and then subjected
to RF heating using a coil surrounding the preform so as to
concentrate heat near its center. This causes partial fusion of the
particulate silica in the inner preform section compared to that in
the outer preform section. Fluorine is diffused into the outer
preform section following densification of the inner preform
section by the RF heating and partial fusion. In other respects the
method is the same as that described with respect to the process
embodiment of FIGS. 2 to 8.
* * * * *